In all weaving, an initially flat array of longitudinally extending warp threads is divided into at least two interspersed groups which are separated in opposite directions from the starting plane to define between the separated warp groups an elongated diamond shaped space, known as a "shed", through which the weft or filling is inserted, the direction of separation of the warp groups being reversed in a given order after each such weft by means of a harness motion with the result that the warp threads are entwined in sinuous fashion around successive filling threads to form the woven fabric. Traditionally, the weft is carried in coiled form upon a bobbin held within a shuttle, and as the weaving progresses, the shuttle is propelled alternatively back and forth through the shed on the upper surface of a beam-like lay which carries a comb-like reed projecting upwardly therefrom and rocks back and forth to press or "beat up" each new weft by means of the reed against the working end or "fell" of the fabric being woven. In the traditional loom, bobbin propulsion was accomplished by means of so-called picker sticks mounted on the loom adjacent opposite side edges of the warp for pivotal movement about their lower ends and driven to alternately impact their upper ends against the shuttle. Obviously, this conventional design was subject to inherent limitations as to achievable shuttle speed and was, moreover, accompanied by substantial disadvantages; namely, deafening operating noise as well as risk of breakage of picker sticks or other damage to equipment and of danger to operating personnel when, as occasionally happened, the shuttle escaped its containment and became an uncontrolled projectile. In order to overcome these inherent problems in bobbin type weaving, the prior art has explored various alternatives, and in the past decade or so, increasing attention has been directed to the possibility of impelling the weft thread through the shed by means of a jet of fluid. Jets of water have been found to be a relatively manageable projection medium, but water is a possible cause of corrosion and limits the choice of yarn material; thus there are significant advantages in the use of a gaseous fluid. While gases other than air can in theory serve equally well, cost considerations dictate the choice of air as the only practical gaseous propelling medium; consequently, this mode of weaving will hereinafter be referred to for convenience as "air weft insertion", although the use instead of other gases is, in principle, intended to be included.
In general, air projection techniques that have been used in past air weft insertion systems fall into two basic categories. In one type, the weft end is initially projected by means of a pressurized air from a nozzle situated outside and adjacent one side of the warp shed which serves to initially accelerate the weft end and starts its travel through the shed. The propulsion forces of existing nozzles is severely limited in terms of the attainable length of projection of the weft end and hence, in this type, a plurality of "booster" or supplemental jet nozzles is provided at spaced intervals through the shed, such nozzles being inserted within and removed in various ways from the shed interior via the clearance between the warp yarns. The aggregate of the propulsion forces of this multi-stage sequence of nozzles can be sufficient to convey the weft thread across the full width of the loom.
While this approach has proved generally feasible in practice, it too is faced with definite disadvantages, viz, the requirement for carefully controlled timing of the sequence of nozzle action plus excessive consumption of compressed air and thus poor economic efficiency.
In order to avoid the need for booster nozzles disposed at intervals through the shed, an alternative approach has been developed in a second type which utilizes a single exterior insertion nozzle in conjunction with a weft guidance "tube" situated within the shed. Since during weaving, the groups of warp threads must shift up and down past one another, the presence of any continuous body within the shed during shedding is out of the question. Therefore, an "interrupted" weft guidance tube is used, taking the form of a plurality of generally annular segments, each shaped to sufficiently narrow thickness in its axial dimension as to pass between adjacent warp threads arranged in an axially aligned position so as to constitute together a lengthwise interrupted tubular member extending substantially the entirety of the shed width. Each annular segment has a slot-like exit opening at a point on its periphery to allow lateral egress of the inserted weft thread when the guidance tube is withdrawn below the shed. When the weft thread is projected by the exterior nozzle into one end of this interrupted guidance tube, the projection force imparted to the thread by the nozzle appears to be substantially enhanced so that the distance the weft thread is propelled by this force can be significantly increased compared to the nozzle alone.
Irrespective of whether the propulsive force of the insertion nozzle is assisted by means of in-shed booster nozzles or an interrupted guidance tube, the control of the flow of the pressurized medium to the injection nozzle presents a certain mechanical problem, due to the inherent characteristics of available flow control valves. In the past, these problems have not been particularly serious because the flow of the insertion medium through the nozzle was maintained for a substantial period of time, relative to the operating cycle of the loom, and it was readily possible to design valve instrumentation that would operate within this relatively broad time frame. However, it can become advantageous from the standpoint of reducing the maximum duration of the operating cycle of the loom so as to achieve a consequential increase in the production speed of the loom to reduce the duration of the flow of the insertion medium through the nozzle; and as the weft insertion time becomes less and less, a limit is reached at which the control of the flow valves becomes quite critical.
In general, the flow valves determining the flow of the insertion medium through the insertion nozzle move between a substantially fully open and a substantially fully closed position so that the insertion medium is either fully admitted to the nozzle or else shut off therefrom, although the extent of the open position may, of course, be adjustable to vary the pressure and quantity of the medium being delivered. Any valve operating in this way has a certain inherent "time constant", i.e. the period of time required for the movable part of the valve to move through an entire cycle of operation and even when the valves are of the electrically actuated solenoid type, and thus free of the inertial and impedance lags of mechanical or pneumatic actuating devices, the time constant of the valve will be a significant value in terms of milliseconds of operating time and as a usual rule, one stage of the valve operating cycle, ordinarily the return phase tends to be considerably longer than the other stage. Consequently, while it might be possible to move the valve, say to open position, in a relatively brief period, the valve must obviously be returned to closed position before it can again be moved to open position and the lag of the solenoid in being restored to starting position constitutes a substantial limit on the practical operating frequency possible with such a valve.